Lift Generation

[SgtPappy]

Wow that's an amazing site. I'll be sure to show my physics teacher. 

[Chalenge]

Most problems visualizing lift are because of associating air with 'nothing' but once you look at air as a 'fluid medium' (similar to water but compressible also) it all begins to make more sense.

[Newman5]

Most problems visualizing lift are because of associating air with 'nothing' but once you look at air as a 'fluid medium' (similar to water but compressible also) it all begins to make more sense.

Agree.

That's why you see scientists and engineers using smoke when when checking drag on an object inside a wind tunnel.

[Die Hard]

The Venturi effect, Bernoulli’s principle and their effect on an airfoil are not just theories, but observable facts:

http://www.youtube.com/watch?v=13eoSasj4hw

http://www.youtube.com/watch?v=6UlsArvbTeo

[Charge]

Defies my logic how a deflected airflow can move faster than undeflected but as is seen in second link it apparently does. So does that mean that air actually "moves faster" through the low pressure area on top of wing?

***

This one is interesting too: http://www.youtube.com/watch?v=QaamEo6WyI4&feature=related

And this: http://www.youtube.com/watch?v=0q3OlZWpOww&feature=related

-C+

[Wmaker]

Some WWII planes could do sort of stalled maneuvers, like the one in your signature; at full power it could hang on propeller, mushing forward 130-140km/h at around 60deg angle (see Kokko's report). Probably some other planes like the P-38 could do similar things. Anyway, the control is probably very limited at such condition.

Yes, I'm aware of the 109's ability to briefly "hang on it's prop".

Harrier style maneuvers does not require ridiculous thrust to weight ratio, just good control at slow speed.

Yep, but like I said, "slowing down to hover" does require thrust to weight over one when it's done without altitude loss and the hover is sustainable, as after all it's called a hover. And thrust to weight over one is "out of this world" when talking about WWII fighters.

As you probably know Turbo Raven had and Turbine Toucan has a very interesting performance. Toucan's t/w is around 1.65 at display weights. Even piston engined (MP-14) Python Pitts has t/w just over one. Interesting planes indeed.

[Wmaker]

About turbo prop biplanes...

Didn't bother to look up the exact t/w on this one but the article sure is a fun read!



http://www.airbum.com/pireps/PirepGreatLksTrbn.html

[CAP1]

Agree.

That's why you see scientists and engineers using smoke when when checking drag on an object inside a wind tunnel.

ACTUALLY,

those of you with r/c models can do that very same thing.

 i did a demo for my cadets, showing them what happens when a wing stalls. i used a diagnostic smoke machine that i use at the shop for finding vacuum leaks. had 2 cadets hold my scale T34 in front of a low speed fan, and i put the tip of the hose infront of the leading edge by a couple inches. they could see the smoke going over and under, comming back together at the trailing edge. then i had them slowly pivot the plane tipping the nose up. at around 17 degrees of so, the smoke got very turbulent going across the top of the wing.
 
 it's a very good visual aid.

[dtango]

Defies my logic how a deflected airflow can move faster than undeflected but as is seen in second link it apparently does. So does that mean that air actually "moves faster" through the low pressure area on top of wing?

It's more intricate than this but yes essentially the air moves faster through the low pressure area on top of the wing.  "Newton" and "Bernoulli" are two different ways of describing how this occurs.  Both are correct.

The bottom line is that an airfoil changes the direction of airflow around it with the result being lift. 

Using Newton (air diversion and reaction) to describe this when you change something's direction you're exerting force which means the wing is exerting force on the air, thus by Newton's F=ma relationship the air must be accelerated.  Since the air is being diverted much more by the upper surface of the wing due to positive aoa the air must experience more acceleration there as well.  The more you divert the air, the more the air is accelerated.

The Bernoulli school of thought looks at this issue through the lens of conservation of mass and energy.  The airfoil changes the direction of the streamtubes of air.  Where you have air streamlines being compressed according to conservation of mass and energy you get lower pressure and higher velocties.  Again thanks to positive aoa the upper surface of the wing changes the air streamline flow more than below and thus higher velocities and lower static pressure.

The key point is that lift is a result of a wing's ability to change the direction of airflow around it which both the Newton and Bernoulli school's of explanation rely on.

Tango, XO
412th FS Braunco Mustangs

[TimRas]

More about downwash and upwash, or why birds fly in a V -formation:
http://www.aerospaceweb.org/question/nature/q0237.shtml

[Charge]

Why I said it defies my logic was that since the airflow above wing is deflected and thus needs to work more (accelerate) to get to the trailing edge it would still be late compared to that "air mass" that started from the leading edge at the same time running the unobstructed lower side thus resulting in a lower pressure above (and aft) of the wing and thus creating "lift". Apparently to video I referred to this is not so, but the pressure dimension and airflow dimension are separate as is also evident in the links I posted which give some explanation to effects of wing profile in drag and lift generation.

I'm fully aware that aerodynamics is not a piece of cake to comprehend but I thought that I'd have such a basic phenomenon as lift generation in my grasp. Well I don't...

-C+

[Gianlupo]

Wow that's an amazing site. I'll be sure to show my physics teacher. 

Which one?

[Stoney]

My personal favorite method with which to consider it, is through the use of symetrical airfoils.  Since both the top and bottom of the airfoil are the same shape and length, you can get away from some of the velocity type definitions.  And, for the same reason, you see that zero degrees angle of attack, the airfoil theoretically produces no lift.  But, create one degree of angle of attack, and you have lift!

So, while it doesn't explain many of the other phenomena associated with the way an airfoil/wing interacts with the relative wind, you can answer Sgt Pappy's original question with a simple "angle of attack".  Its generalistic and brushes over a few other facets of airfoil theory, but in a single sentence, it sums it up nicely, IMHO...

[Die Hard]

My personal favorite method with which to consider it, is through the use of symetrical airfoils.  Since both the top and bottom of the airfoil are the same shape and length, you can get away from some of the velocity type definitions.  And, for the same reason, you see that zero degrees angle of attack, the airfoil theoretically produces no lift.  But, create one degree of angle of attack, and you have lift!

So, while it doesn't explain many of the other phenomena associated with the way an airfoil/wing interacts with the relative wind, you can answer Sgt Pappy's original question with a simple "angle of attack".  Its generalistic and brushes over a few other facets of airfoil theory, but in a single sentence, it sums it up nicely, IMHO...

No, not really. The more the angle of attack, the more of the rounded leading edge curves to the top of the airfoil, and the flatter the bottom of the airfoil. Simply by changing the angle of attack you also change the effective shape of the airfoil.

[Stoney]

No, not really.

Yes really.  Sgt Pappy's initial question was "what creates lift?"  My answer was "angle of attack".  And you cannot change the "effective" shape of the airfoil by increasing angle of attack.  Airfoil theory is probably best served in another thread.